[PATCH 38/46] mm/ksm: reset the unstable trees of all nodes

[xu . xin]

> 
> Let's fix it by tracking in ksm_nr_node_ids the actual number of
> allocated tree roots.
> 
> remove_all_stable_nodes() and ksm_check_stable_tree() will now scan
> more tree roots (skipping the empty ones), which is harmless.
> 
> Fixes: ef53d16cded7 ("ksm: allocate roots when needed")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> ---
>  mm/ksm.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index dbfb8f98b1892..9c51925825cbf 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -475,6 +475,7 @@ static void advisor_stop_scan(void)
>  #ifdef CONFIG_NUMA
>  /* Zeroed when merging across nodes is not allowed */
>  static unsigned int ksm_merge_across_nodes = 1;
> +/* Number of allocated stable and unstable tree roots. */
>  static int ksm_nr_node_ids = 1;
>  #else
>  #define ksm_merge_across_nodes	1U
> @@ -3636,14 +3637,13 @@ static ssize_t merge_across_nodes_store(struct kobject *kobj,
>  			else {
>  				root_stable_tree = buf;
>  				root_unstable_tree = buf + nr_node_ids;
> +				ksm_nr_node_ids = nr_node_ids;
>  				/* Stable tree is empty but not the unstable */
>  				root_unstable_tree[0] = one_unstable_tree[0];
>  			}
>  		}
> -		if (!err) {
> +		if (!err)
>  			ksm_merge_across_nodes = knob;
> -			ksm_nr_node_ids = knob ? 1 : nr_node_ids;
> -		}
>  	}
>  	mutex_unlock(&ksm_thread_mutex);
>  
> 
> -- 
> 2.43.0

Looks correct as far as it goes, but I think it is not quite sufficient
on its own.

The documentation requires that merge_across_nodes can only be changed
when there are no KSM shared pages in the system, but
merge_across_nodes_store() does not enforce this at the sysfs level.

Should we also reject writes while ksmd is merging? At minimum:

+	if (ksm_run & KSM_RUN_MERGE)
+		return -EBUSY;

Ideally, we should also check that ksm_pages_shared is zero, since
KSM_RUN_STOP (0) does not guarantee that all pages have been unmerged.

That would make the interface match the documented sequence and close
the remaining race window.